1966 390 low vacuum at manifold and poor idle when warm

Sun lover

New Member
Joined
Nov 21, 2021
Thunderbird Year
1966
I recently purchased a 66 convertible with a 390. It starts ok and idles well at cold idle but when at temperature, the idle is erratic and sometimes, the engine will barely keep running. I found and fixed several vacuum leaks but still cannot get the vacuum above 14 inches. Adjusting the idle screws on the carb seems to make no difference in vacuum or idle quality. The carb is clean and shiny which makes me think that it has recently been rebuilt or replaced. Any ideas other than replacing the carb? I have also noticed that when I step on the brakes the idle speeds up slightly however, blocking off the vacuum line to the brakes did not improve my low vacuum situation.
 
Joined
Aug 14, 2019
Thunderbird Year
1957
I recently purchased a 66 convertible with a 390. It starts ok and idles well at cold idle but when at temperature, the idle is erratic and sometimes, the engine will barely keep running. I found and fixed several vacuum leaks but still cannot get the vacuum above 14 inches. Adjusting the idle screws on the carb seems to make no difference in vacuum or idle quality. The carb is clean and shiny which makes me think that it has recently been rebuilt or replaced. Any ideas other than replacing the carb? I have also noticed that when I step on the brakes the idle speeds up slightly however, blocking off the vacuum line to the brakes did not improve my low vacuum situation.
A long shot but a can of spray carb cleaner goes a long ways in these situations. Sometimes it can be as simple as the carb to manifold junction.
Spray the cleaner at all vacuum junctions and many times it will reveal the vacuum leak when the idle improves when it hits the leak and it won't harm anything.
Oh and don't forget the climate controls. Lots of vacuum lines there too. Ford still loved vacuum controls before electric servos were reliable.
 

Sun lover

New Member
Joined
Nov 21, 2021
Thunderbird Year
1966
I have lifted and blocked all vacuum lines except the vacuum advance and the PC valve (which tested good) and sprayed carb cleaner around the base of the carburetor. I did not notice any change in idle speed. The engine has 74,000 miles on it and does not appear to have been apart. is it possible that it is leaking somewhere between the manifold and the heads? (it does run hotter than I like, it has a 190 thermostat and tends to run around 210 degrees).
 
Joined
Aug 14, 2019
Thunderbird Year
1957
I have lifted and blocked all vacuum lines except the vacuum advance and the PC valve (which tested good) and sprayed carb cleaner around the base of the carburetor. I did not notice any change in idle speed. The engine has 74,000 miles on it and does not appear to have been apart. is it possible that it is leaking somewhere between the manifold and the heads? (it does run hotter than I like, it has a 190 thermostat and tends to run around 210 degrees).
My theory still stands. Spray around the manifold to head. It's quite possible that that could be your leak. As others have suggested some chemicals that will refresh the seals may help. Not permanent but to make it useable. As many will attest on here a season or two won't do any harm but years of sitting is a whole 'nother ball of wax.
 

74 Harley

Active Member
Joined
Jul 8, 2020
Thunderbird Year
1962
@Ward 57 , while I do agree with your method, the manifold to head sealing area is under the valve covers.
Since the carburetor appears to have been replaced is it correct? It sounds like it is possibly running rich if idle improves momentarily with brake application.
The carburetor may need to be jetted down if it was replaced.
 
Joined
Feb 9, 2019
Thunderbird Year
2002
If the car runs good at a fast idle and with choke enriching the mixture, you may be fuel-starved or running lean when the engine is warm.. I would first check the initial timing and vacuum advance operation then delve into the fuel system.
If the idle mixture screws are not changing anything that's an indication of a problem...often increased timing advance or other isseus can have you "chasing" a clean idle with the adjustments until you are over-exposing the "transfer slot" in the carb throat, when this happens the screws no longer have any effect. Here is an example of an over-exposed transfer slot. transfer_slot.jpg

Normally with the throttle plates closed that slot should be nearly square and not a rectangle. In addition, you should check the fuel pump pressure and volume, any vacuum gauge is also a fuel pump pressure tester and the instructions will indicate how to perform the test,.
 

64ZCODE

Active Member
Joined
Aug 18, 2018
Thunderbird Year
1964
I have lifted and blocked all vacuum lines except the vacuum advance and the PC valve (which tested good) and sprayed carb cleaner around the base of the carburetor. I did not notice any change in idle speed. The engine has 74,000 miles on it and does not appear to have been apart. is it possible that it is leaking somewhere between the manifold and the heads? (it does run hotter than I like, it has a 190 thermostat and tends to run around 210 degrees).
Recommend capping off all connections to manifold vacuum, including to vacuum advance and to the PCV valve as well as to the power brake booster and to interior cabin controls and switches, then check manifold vacuum again. If it's still less than 16-17 inches of mercury, then try a different vacuum gauge and see if the result is the same. If it is, you likely have worn rings or a stuck valve. If a particular cylinder is the culprit, then you should see the needle on the vacuum gauge oscillate when the engine is idling.
 
Joined
Sep 5, 2018
Thunderbird Year
2005
I recently purchased a 66 convertible with a 390. It starts ok and idles well at cold idle but when at temperature, the idle is erratic and sometimes, the engine will barely keep running. I found and fixed several vacuum leaks but still cannot get the vacuum above 14 inches. Adjusting the idle screws on the carb seems to make no difference in vacuum or idle quality. The carb is clean and shiny which makes me think that it has recently been rebuilt or replaced. Any ideas other than replacing the carb? I have also noticed that when I step on the brakes the idle speeds up slightly however, blocking off the vacuum line to the brakes did not improve my low vacuum situation.
Hi, There are a number of conditions that could be the culprit causing your low vacuum. You did well finding what you found. Here are steps I would take. First 14“ is too low for power Brakes. As an old PA state inspection mechanic (who also specialized in Fords), 18” is my preferred minimum In these cars. That said I’d be interested in knowing how hard you must depress the brake pedal to stop? I assume your vacuum gauge is accurate? if you are not sure, test it on another car Or borrow a friends gauge. Assuming the gauge is fine I would start as follows:
1. unhook and plug every vacuum line. Pay particular attention to the one that exits the intake manifold and supplies the automatic trans. Some of these cars have two,pieces. One at the back of the intake manifold and a smaller one at the modulator valve on the trans. To access that one you need a lift or ramps to get under the car. It is located near the rear end of the trans.
2) Does the trans shift okay? if any part of any hose is bad it will act like the shift is delayed or not shift at all.
3) look for any tiny 1/8 ” hoses, some of which go through the firewall into the floor of the passenger side. They work the heat/a/c doors.
4) if the distributor is vacuum assisted obviously check that too. You may have a distributor with mechanical advance only in which case there will,be no vacuum hose going to it.
5) with every line removed and plugged put the gauge on the manifold. What’s the reading now?
6) if it is still at 14 I would expect an intake manifold leak either at the carb gasket or at he heads. You can pull off the carb and change the gasket easily enough. of course the carb could be bad and Need to be rebuilt.
7) The vacuum booster could be leaking but since you said disconnecting it didn’t help I will assume you are satisfied there.
8)Some cars have a vacuum cannister buried somewhere in the engine compartment or passenger wheel well. Ford used metal cans that looked like a large soup can . They would stuff them inside the wheel well of the passenger side by the hood hinge and over the years they rot out but you can’t see it. Make sure if you have one, to disconnect that vacuum line from the intake usually off the same manifold tap as the power brake booster. also the brake booster has a one way valve. If it leaks you lose vacuum. So you must plug the hose before it enters the valve on the booster to test the booster and valve.
9)The final thing would be the most expensive to repair. That is a burnt valve. 390’s are notorious for burning a valve. To find out if this is the case you would remove all the spark plugs and do a compression test. Hopefully you have access to a tester. The general rule is that the cylinders should all read a minimum of 100 lbs or better with no more than a 10% difference between the highest and lowest.
Depending on your skill level you may need to have a qualified mechanic do some of this work. Good luck and I hope it’s isn’t #9.
Good luck and Happy Thanksgiving!
Probably more info than you want but that’s everything it would take to find the source of the leak.
 

Sun lover

New Member
Joined
Nov 21, 2021
Thunderbird Year
1966
Hi, There are a number of conditions that could be the culprit causing your low vacuum. You did well finding what you found. Here are steps I would take. First 14“ is too low for power Brakes. As an old PA state inspection mechanic (who also specialized in Fords), 18” is my preferred minimum In these cars. That said I’d be interested in knowing how hard you must depress the brake pedal to stop? I assume your vacuum gauge is accurate? if you are not sure, test it on another car Or borrow a friends gauge. Assuming the gauge is fine I would start as follows:
1. unhook and plug every vacuum line. Pay particular attention to the one that exits the intake manifold and supplies the automatic trans. Some of these cars have two,pieces. One at the back of the intake manifold and a smaller one at the modulator valve on the trans. To access that one you need a lift or ramps to get under the car. It is located near the rear end of the trans.
2) Does the trans shift okay? if any part of any hose is bad it will act like the shift is delayed or not shift at all.
3) look for any tiny 1/8 ” hoses, some of which go through the firewall into the floor of the passenger side. They work the heat/a/c doors.
4) if the distributor is vacuum assisted obviously check that too. You may have a distributor with mechanical advance only in which case there will,be no vacuum hose going to it.
5) with every line removed and plugged put the gauge on the manifold. What’s the reading now?
6) if it is still at 14 I would expect an intake manifold leak either at the carb gasket or at he heads. You can pull off the carb and change the gasket easily enough. of course the carb could be bad and Need to be rebuilt.
7) The vacuum booster could be leaking but since you said disconnecting it didn’t help I will assume you are satisfied there.
8)Some cars have a vacuum cannister buried somewhere in the engine compartment or passenger wheel well. Ford used metal cans that looked like a large soup can . They would stuff them inside the wheel well of the passenger side by the hood hinge and over the years they rot out but you can’t see it. Make sure if you have one, to disconnect that vacuum line from the intake usually off the same manifold tap as the power brake booster. also the brake booster has a one way valve. If it leaks you lose vacuum. So you must plug the hose before it enters the valve on the booster to test the booster and valve.
9)The final thing would be the most expensive to repair. That is a burnt valve. 390’s are notorious for burning a valve. To find out if this is the case you would remove all the spark plugs and do a compression test. Hopefully you have access to a tester. The general rule is that the cylinders should all read a minimum of 100 lbs or better with no more than a 10% difference between the highest and lowest.
Depending on your skill level you may need to have a qualified mechanic do some of this work. Good luck and I hope it’s isn’t #9.
Good luck and Happy Thanksgiving!
Probably more info than you want but that’s everything it would take to find the source of the leak.
Thanks for all the info, this is My first ford but I have had a number of GMs (it is nice to have the distributor in front for a change!) I did a compression test and the average is 125 but #2 read 150 (it is not easy to test on a thunderbird with air!) All of the plugs indicate that the car is running rich and I have no idea of the quality of the rebuild on the carburetor. I have never had much luck rebuilding carburetors and I have been hoping to find the problem elsewhere rather than spending money on a replacement which is like troubleshooting with a shotgun! Rearging the power brakes, it does take more force than it should to stop the car, probably because of the low vacuum
Hi, There are a number of conditions that could be the culprit causing your low vacuum. You did well finding what you found. Here are steps I would take. First 14“ is too low for power Brakes. As an old PA state inspection mechanic (who also specialized in Fords), 18” is my preferred minimum In these cars. That said I’d be interested in knowing how hard you must depress the brake pedal to stop? I assume your vacuum gauge is accurate? if you are not sure, test it on another car Or borrow a friends gauge. Assuming the gauge is fine I would start as follows:
1. unhook and plug every vacuum line. Pay particular attention to the one that exits the intake manifold and supplies the automatic trans. Some of these cars have two,pieces. One at the back of the intake manifold and a smaller one at the modulator valve on the trans. To access that one you need a lift or ramps to get under the car. It is located near the rear end of the trans.
2) Does the trans shift okay? if any part of any hose is bad it will act like the shift is delayed or not shift at all.
3) look for any tiny 1/8 ” hoses, some of which go through the firewall into the floor of the passenger side. They work the heat/a/c doors.
4) if the distributor is vacuum assisted obviously check that too. You may have a distributor with mechanical advance only in which case there will,be no vacuum hose going to it.
5) with every line removed and plugged put the gauge on the manifold. What’s the reading now?
6) if it is still at 14 I would expect an intake manifold leak either at the carb gasket or at he heads. You can pull off the carb and change the gasket easily enough. of course the carb could be bad and Need to be rebuilt.
7) The vacuum booster could be leaking but since you said disconnecting it didn’t help I will assume you are satisfied there.
8)Some cars have a vacuum cannister buried somewhere in the engine compartment or passenger wheel well. Ford used metal cans that looked like a large soup can . They would stuff them inside the wheel well of the passenger side by the hood hinge and over the years they rot out but you can’t see it. Make sure if you have one, to disconnect that vacuum line from the intake usually off the same manifold tap as the power brake booster. also the brake booster has a one way valve. If it leaks you lose vacuum. So you must plug the hose before it enters the valve on the booster to test the booster and valve.
9)The final thing would be the most expensive to repair. That is a burnt valve. 390’s are notorious for burning a valve. To find out if this is the case you would remove all the spark plugs and do a compression test. Hopefully you have access to a tester. The general rule is that the cylinders should all read a minimum of 100 lbs or better with no more than a 10% difference between the highest and lowest.
Depending on your skill level you may need to have a qualified mechanic do some of this work. Good luck and I hope it’s isn’t #9.
Good luck and Happy Thanksgiving!
Probably more info than you want but that’s everything it would take to find the source of t
 
Joined
Aug 28, 2021
Thunderbird Year
1961
Thanks for all the info, this is My first ford but I have had a number of GMs (it is nice to have the distributor in front for a change!) I did a compression test and the average is 125 but #2 read 150 (it is not easy to test on a thunderbird with air!) All of the plugs indicate that the car is running rich and I have no idea of the quality of the rebuild on the carburetor. I have never had much luck rebuilding carburetors and I have been hoping to find the problem elsewhere rather than spending money on a replacement which is like troubleshooting with a shotgun! Rearging the power brakes, it does take more force than it should to stop the car, probably because of the low vacuum
Maybe the timing chain has jumped one tooth and the previous owner adjusted the distributor to compensate for that or the harmonic balancer has slipped and Your timing mark is incorrect
 

74 Harley

Active Member
Joined
Jul 8, 2020
Thunderbird Year
1962
Here's a couple of things you can try.
First check the timing chain.( Much easier than you think) remove the distributor cap. Use a breaker bar and roll the engine to tdc. ( Doesn't matter which way you go)
Now watch the rotor and roll the engine in the opposite direction. Stop as soon as you see the rotor move. Read where the timing mark is and that is the amount of slack in you timing chain. replacement is recommended at 10 degrees.
Next up, warm up the car. While at an idle remove a small vacuum hose. ( Not vacuum advance). Does the idle improve? If so you may need smaller jets in the carburetor.
 
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